Several lines of evidence show that endogenous and exogenous cannabinoids modulate pain transmission at the spinal level through specific cannabinoid-1 (CB1) receptors. Since anatomical data concerning spinal CB1 receptors are rather contradictory, we studied the cellular and subcellular localizations of the CB1 receptors by immunocytochemistry. Results show a dual pre- and postsynaptic localization of CB1 receptors. Presynaptic receptors are evidenced by the labeling of (1) heterogeneous dorsal root ganglion neurons and (2) axons of Lissauer's tract. Postsynaptic receptors are shown by the labeling of numerous interneurons in the outer part of lamina II. Double immunolabelings show that lamina II outer CB1 neurons, probably islet cells, may also contain GABA or nitric oxide synthase. Numerous CB1-containing neurons in lamina X are also immunostained with anti-nitric oxide synthase (NOS) antibody. Under the electron microscope, CB1 immunoreactivity is exclusively localized postsynaptically in both somatic and dendritic compartments. The absence of labeling on primary afferent axon terminals is discussed and compared to the absence of labeling on terminals or vesicle-containing dendrites of islet cells, where a presynaptic localization was expected according to data of the literature.